exhausted simplifier ticks and hs-boot files

[Simon Peyton Jones]

Ganesh

It's an old bug that has somehow reappeared.
I opened https://gitlab.haskell.org/ghc/ghc/issues/17140

But it seems OK in HEAD, and hence probably in GHC 8.8.  Can you try?

Maybe put further comments on the issue tracker, rather than here.

thanks

Simon

| -----Original Message-----
| From: Glasgow-haskell-users <glasgow-haskell-users-bounces at haskell.org> On
| Behalf Of Ganesh Sittampalam
| Sent: 05 August 2019 14:02
| To: glasgow-haskell-users at haskell.org
| Subject: exhausted simplifier ticks and hs-boot files
| 
| Hi,
| 
| The code below (also attached - unzip and run go.sh) triggers the GHC
| panic "Simplifier ticks exhausted", and I'm unsure whether I should view
| it as an instance of the known infelicity in the inliner
| (https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/bugs.html
| #bugs-ghc)
| 
| My code does have a recursive datatype, but the recursion isn't
| contravariant, which is the case described in "Secrets of the GHC inliner"
| (https://www.microsoft.com/en-us/research/wp-
| content/uploads/2002/07/inline.pdf,
| section 4).
| 
| It's cut down from some real code where I have a mutually recursive
| datatype that I want to define across two modules for code structuring
| reasons, meaning I need a .hs-boot file. I haven't been able to reproduce
| it without the .hs-boot file - if I put everything in one module it stops
| happening.
| 
| I've tried with a range of GHC versions from 8.2.x to an early version of
| 8.8. It happens with -O1 and not -O0, but I haven't managed to find a
| specific optimisation that triggers it.
| 
| Is this just an instance of the known problem in a different guise, or is
| it something different and worth a bug report?
| 
| Cheers,
| 
| Ganesh
| 
| T2.hs-boot
| -----------
| module T2 where
| 
| data T2
| 
| mapP_T2 :: (Int -> Int) -> T2 -> T2
| 
| T1.hs
| -----
| module T1 where
| 
| import {-# SOURCE #-} T2
| 
| data T1 = T1 T2
| 
| mapP_T1 :: (Int -> Int) -> T1 -> T1
| mapP_T1 _ (T1 xs) = T1 (mapP_T2 id xs)
| 
| T2.hs
| -----
| 
| module T2 where
| 
| import T1
| 
| data T2 = T2 T1
| 
| mapP_T2 :: (Int -> Int) -> T2 -> T2
| mapP_T2 f (T2 t) = T2 (mapP_T1 f t)
| 
| go :: T1 -> T1
| go = mapP_T1 id
| 
| GHC output
| ----------
| $ ghc --make T2.hs -O1 -fsimpl-tick-factor=1000 -ddump-simpl-stats) [...]
| ghc.exe: panic! (the 'impossible' happened)
|   (GHC version 8.2.2 for x86_64-unknown-mingw32):
|         Simplifier ticks exhausted
|   When trying UnfoldingDone mapP_T2
|   To increase the limit, use -fsimpl-tick-factor=N (default 100)
|   If you need to do this, let GHC HQ know, and what factor you needed
|   Total ticks:     61203
| 
|   24481 PreInlineUnconditionally
|     6121 ds_i17h
|     6120 f_a16p
|     6120 ds_d17d
|     6120 ds1_i17i
|   12241 UnfoldingDone
|     6121 mapP_T1
|     6120 mapP_T2
|   24481 BetaReduction
|     6121 ds_i17h
|     6120 f_a16p
|     6120 ds_d17d
|     6120 ds1_i17i